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arXiv · 1607.08258

Nordhaus-Gaddum and other bounds for the sum of squares of the positive eigenvalues of a graph

Clive Elphick·Mustapha Aouchiche
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Abstract

Terpai [22] proved the Nordhaus-Gaddum bound that $\mu(G) + \mu(\overline{G}) \le 4n/3 - 1$, where $\mu(G)$ is the spectral radius of a graph $G$ with $n$ vertices. Let $s^+$ denote the sum of the squares of the positive eigenvalues of $G$. We prove that $\sqrt{s^{+}(G)} + \sqrt{s^+(\overline{G})} < \sqrt{2}n$ and conjecture that $\sqrt{s^{+}(G)} + \sqrt{s^+(\overline{G})} \le 4n/3 - 1.$ We have used AutoGraphiX and Wolfram Mathematica to search for a counter-example. We also consider Nordhaus-Gaddum bounds for $s^+$ and bounds for the Randi\'c index.

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Clive Elphick, Mustapha Aouchiche. 2016-07-27. Nordhaus-Gaddum and other bounds for the sum of squares of the positive eigenvalues of a graph. https://arxiv.org/abs/1607.08258

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